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<a class="nav-link dropdown-toggle jmu-gold rounded" href="POSIXThreads.html#" id="navbarDropdownChapters" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Contents</a>
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<a class="dropdown-item" tabindex="-1" href="POSIXThreads.html#"><b>Chapter 1</b></a>
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<a class="dropdown-item" href="IntroConcSysOverview.html"> 1.1. Introduction to Concurrent Systems</a>
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<a class="dropdown-item" href="SysAndModels.html"> 1.2. Systems and Models</a>
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<a class="dropdown-item" href="Themes.html"> 1.3. Themes and Guiding Principles</a>
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<a class="dropdown-item" href="StateModels.html"> 1.5. State Models in UML</a>
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<a class="dropdown-item" href="SequenceModels.html"> 1.6. Sequence Models in UML</a>
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<a class="dropdown-item" href="StateModelImplementation.html"> 1.7. Extended Example: State Model Implementation</a>
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<a class="dropdown-item disabled"><b>Chapter 2</b></a>
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<a class="dropdown-item" href="ProcessesOverview.html"> 2.1. Processes and OS Basics</a>
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<a class="dropdown-item" href="Multiprogramming.html"> 2.2. Processes and Multiprogramming</a>
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<a class="dropdown-item" href="KernelMechanics.html"> 2.3. Kernel Mechanics</a>
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<a class="dropdown-item" href="Syscall.html"> 2.4. System Call Interface</a>
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<a class="dropdown-item" href="ProcessCycle.html"> 2.5. Process Life Cycle</a>
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<a class="dropdown-item" href="UnixFile.html"> 2.6. The UNIX File Abstraction</a>
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<a class="dropdown-item" href="EventsSignals.html"> 2.7. Events and Signals</a>
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<a class="dropdown-item" href="Extended2Processes.html"> 2.8. Extended Example: Listing Files with Processes</a>
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<a class="dropdown-item disabled"><b>Chapter 3</b></a>
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<a class="dropdown-item" href="IPCOverview.html"> 3.1. Concurrency with IPC</a>
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<a class="dropdown-item" href="IPCModels.html"> 3.2. IPC Models</a>
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<a class="dropdown-item" href="Pipes.html"> 3.3. Pipes and FIFOs</a>
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<a class="dropdown-item" href="MMap.html"> 3.4. Shared Memory With Memory-mapped Files</a>
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<a class="dropdown-item" href="POSIXvSysV.html"> 3.5. POSIX vs. System V IPC</a>
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<a class="dropdown-item" href="MQueues.html"> 3.6. Message Passing With Message Queues</a>
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<a class="dropdown-item" href="ShMem.html"> 3.7. Shared Memory</a>
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<a class="dropdown-item" href="IPCSems.html"> 3.8. Semaphores</a>
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<a class="dropdown-item" href="Extended3Bash.html"> 3.9. Extended Example: Bash-lite: A Simple Command-line Shell</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 4</b></a>
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<a class="dropdown-item" href="SocketsOverview.html"> 4.1. Networked Concurrency</a>
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<a class="dropdown-item" href="FiveLayer.html"> 4.2. The TCP/IP Internet Model</a>
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<a class="dropdown-item" href="NetApps.html"> 4.3. Network Applications and Protocols</a>
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<a class="dropdown-item" href="Sockets.html"> 4.4. The Socket Interface</a>
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<a class="dropdown-item" href="TCPSockets.html"> 4.5. TCP Socket Programming: HTTP</a>
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<a class="dropdown-item" href="UDPSockets.html"> 4.6. UDP Socket Programming: DNS</a>
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<a class="dropdown-item" href="AppBroadcast.html"> 4.7. Application-Layer Broadcasting: DHCP</a>
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<a class="dropdown-item" href="Extended4CGI.html"> 4.8. Extended Example: CGI Web Server</a>
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<a class="dropdown-item disabled"><b>Chapter 5</b></a>
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<a class="dropdown-item" href="InternetOverview.html"> 5.1. The Internet and Connectivity</a>
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<a class="dropdown-item" href="AppLayer.html"> 5.2. Application Layer: Overlay Networks</a>
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<a class="dropdown-item" href="TransLayer.html"> 5.3. Transport Layer</a>
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<a class="dropdown-item" href="NetSec.html"> 5.4. Network Security Fundamentals</a>
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<a class="dropdown-item" href="NetLayer.html"> 5.5. Network Layer: IP</a>
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<a class="dropdown-item" href="LinkLayer.html"> 5.6. Link Layer</a>
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<a class="dropdown-item" href="Wireless.html"> 5.7. Wireless Connectivity: Wi-Fi, Bluetooth, and Zigbee</a>
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<a class="dropdown-item" href="Extended5DNS.html"> 5.8. Extended Example: DNS client</a>
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<a class="dropdown-item disabled"><b>Chapter 6</b></a>
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<a class="dropdown-item" href="ThreadsOverview.html"> 6.1. Concurrency with Multithreading</a>
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<a class="dropdown-item" href="ProcVThreads.html"> 6.2. Processes vs. Threads</a>
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<a class="dropdown-item" href="RaceConditions.html"> 6.3. Race Conditions and Critical Sections</a>
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<a class="dropdown-item" href="POSIXThreads.html"> 6.4. POSIX Thread Library</a>
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<a class="dropdown-item" href="ThreadArgs.html"> 6.5. Thread Arguments and Return Values</a>
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<a class="dropdown-item" href="ImplicitThreads.html"> 6.6. Implicit Threading and Language-based Threads</a>
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<a class="dropdown-item" href="Extended6Input.html"> 6.7. Extended Example: Keyboard Input Listener</a>
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<a class="dropdown-item" href="Extended6Primes.html"> 6.8. Extended Example: Concurrent Prime Number Search</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 7</b></a>
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<a class="dropdown-item" href="SynchOverview.html"> 7.1. Synchronization Primitives</a>
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<a class="dropdown-item" href="CritSect.html"> 7.2. Critical Sections and Peterson's Solution</a>
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<a class="dropdown-item" href="Locks.html"> 7.3. Locks</a>
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<a class="dropdown-item" href="Semaphores.html"> 7.4. Semaphores</a>
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<a class="dropdown-item" href="Barriers.html"> 7.5. Barriers</a>
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<a class="dropdown-item" href="Condvars.html"> 7.6. Condition Variables</a>
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<a class="dropdown-item" href="Deadlock.html"> 7.7. Deadlock</a>
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<a class="dropdown-item" href="Extended7Events.html"> 7.8. Extended Example: Event Log File</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 8</b></a>
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<a class="dropdown-item" href="SynchProblemsOverview.html"> 8.1. Synchronization Patterns and Problems</a>
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<a class="dropdown-item" href="SynchDesign.html"> 8.2. Basic Synchronization Design Patterns</a>
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<a class="dropdown-item" href="ProdCons.html"> 8.3. Producer-Consumer Problem</a>
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<a class="dropdown-item" href="ReadWrite.html"> 8.4. Readers-Writers Problem</a>
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<a class="dropdown-item" href="DiningPhil.html"> 8.5. Dining Philosophers Problem and Deadlock</a>
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<a class="dropdown-item" href="CigSmokers.html"> 8.6. Cigarette Smokers Problem and the Limits of Semaphores and Locks</a>
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<a class="dropdown-item" href="Extended8ModExp.html"> 8.7. Extended Example: Parallel Modular Exponentiation</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 9</b></a>
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<a class="dropdown-item" href="ParallelDistributedOverview.html"> 9.1. Parallel and Distributed Systems</a>
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<a class="dropdown-item" href="ParVConc.html"> 9.2. Parallelism vs. Concurrency</a>
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<a class="dropdown-item" href="ParallelDesign.html"> 9.3. Parallel Design Patterns</a>
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<a class="dropdown-item" href="Scaling.html"> 9.4. Limits of Parallelism and Scaling</a>
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<a class="dropdown-item" href="DistTiming.html"> 9.5. Timing in Distributed Environments</a>
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<a class="dropdown-item" href="DistDataStorage.html"> 9.6. Reliable Data Storage and Location</a>
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<a class="dropdown-item" href="DistConsensus.html"> 9.7. Consensus in Distributed Systems</a>
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<a class="dropdown-item" href="Extended9Blockchain.html"> 9.8. Extended Example: Blockchain Proof-of-Work</a>
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<a class="dropdown-item disabled"><b>Appendix A</b></a>
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<a class="dropdown-item" href="CLangOverview.html"> A.1. C Language Reintroduction</a>
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<a class="dropdown-item" href="Debugging.html"> A.2. Documentation and Debugging</a>
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<a class="dropdown-item" href="BasicTypes.html"> A.3. Basic Types and Pointers</a>
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<a class="dropdown-item" href="Arrays.html"> A.4. Arrays, Structs, Enums, and Type Definitions</a>
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<a class="dropdown-item" href="Functions.html"> A.5. Functions and Scope</a>
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<a class="dropdown-item" href="Pointers.html"> A.6. Pointers and Dynamic Allocation</a>
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<a class="dropdown-item" href="Strings.html"> A.7. Strings</a>
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<a class="dropdown-item" href="FunctionPointers.html"> A.8. Function Pointers</a>
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<a class="dropdown-item" href="Files.html"> A.9. Files</a>
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</div>
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<li class="nav-item"><a class="nav-link jmu-gold" href="https://w3.cs.jmu.edu/kirkpams/OpenCSF/Books/csf/source/POSIXThreads.rst"
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«  <a id="prevmod" href="RaceConditions.html">6.3. Race Conditions and Critical Sections</a>
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  ::  
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<a class="uplink" href="index.html">Contents</a>
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  ::  
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<a id="nextmod" href="ThreadArgs.html">6.5. Thread Arguments and Return Values</a>  »
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<script>ODSA.SETTINGS.DISP_MOD_COMP = true;ODSA.SETTINGS.MODULE_NAME = "POSIXThreads";ODSA.SETTINGS.MODULE_LONG_NAME = "POSIX Thread Library";ODSA.SETTINGS.MODULE_CHAPTER = "Concurrency with Multithreading"; ODSA.SETTINGS.BUILD_DATE = "2021-06-11 17:38:24"; ODSA.SETTINGS.BUILD_CMAP = false;JSAV_OPTIONS['lang']='en';JSAV_EXERCISE_OPTIONS['code']='java';</script><div class="section" id="posix-thread-library">
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<h1>6.4. POSIX Thread Library<a class="headerlink" href="POSIXThreads.html#posix-thread-library" title="Permalink to this headline">¶</a></h1>
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<p>In this section, we’ll return to the code from <a class="reference external" href="ProcVThreads.html#cl6-1">Code Listing 6.1</a> and replace the comments with functions from the
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<a class="reference internal" href="Glossary.html#term-posix-thread-library"><span class="xref std std-term">POSIX thread library (pthreads)</span></a>. Pthread implementations
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are available for a wide variety of platforms, including Linux, macOS, and
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Windows. After considering the basic techniques to get threads started, we’ll
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look at how to pass arguments to and return data from the thread.</p>
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<div class="section" id="creating-and-joining-threads">
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<h2>6.4.1. Creating and Joining Threads<a class="headerlink" href="POSIXThreads.html#creating-and-joining-threads" title="Permalink to this headline">¶</a></h2>
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<p>Three functions define the core functionality for creating and managing threads.
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The <code class="docutils literal notranslate"><span class="pre">pthread_create()</span></code> function will create and start a new thread inside a
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process. The <code class="docutils literal notranslate"><span class="pre">start_routine</span></code> parameter specifies the name of the function to
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use as the thread’s entry point, just as <code class="docutils literal notranslate"><span class="pre">main()</span></code> serves as the main thread’s
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entry point. The pthread_exit() is used to exit the current thread and
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optionally return a value. Finally, the <code class="docutils literal notranslate"><span class="pre">pthread_join()</span></code> function is the
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thread equivalent of the <code class="docutils literal notranslate"><span class="pre">wait()</span></code> function for processes. That is, calling
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<code class="docutils literal notranslate"><span class="pre">pthread_join()</span></code> on a child thread will cause the current (parent) thread to
|
|||
|
|
wait until the child finishes and calls <code class="docutils literal notranslate"><span class="pre">pthread_exit()</span></code>.</p>
|
|||
|
|
<div class="topic border border-dark rounded-lg bg-light px-2 mb-3">
|
|||
|
|
<div class="figure align-left">
|
|||
|
|
<a class="reference internal image-reference" href="_images/CSF-Images-Library.png"><img alt="Decorative C library image" src="_images/CSF-Images-Library.png" style="width: 100%;" /></a>
|
|||
|
|
</div>
|
|||
|
|
<p class="topic-title first pt-2 mb-1">C library functions – <pthread.h></p><hr class="mt-1" />
|
|||
|
|
<dl class="docutils">
|
|||
|
|
<dt><code class="docutils literal notranslate"><span class="pre">int</span> <span class="pre">pthread_create</span> <span class="pre">(pthread_t</span> <span class="pre">*thread,</span> <span class="pre">const</span> <span class="pre">pthread_attr_t</span> <span class="pre">*attr,</span> <span class="pre">void</span> <span class="pre">*(*start_routine)(void*),</span> <span class="pre">void</span> <span class="pre">*arg);</span></code></dt>
|
|||
|
|
<dd>Create a new thread starting with at the <code class="docutils literal notranslate"><span class="pre">start_routine</span></code> function.</dd>
|
|||
|
|
<dt><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">pthread_exit</span> <span class="pre">(void</span> <span class="pre">*value_ptr);</span></code></dt>
|
|||
|
|
<dd>Exit from the current thread.</dd>
|
|||
|
|
<dt><code class="docutils literal notranslate"><span class="pre">int</span> <span class="pre">pthread_join</span> <span class="pre">(pthread_t</span> <span class="pre">thread,</span> <span class="pre">void</span> <span class="pre">**value_ptr);</span></code></dt>
|
|||
|
|
<dd>Join a specified thread.</dd>
|
|||
|
|
</dl>
|
|||
|
|
</div>
|
|||
|
|
<p>The arg argument to the <code class="docutils literal notranslate"><span class="pre">pthread_create()</span></code> function gets passed verbatim as
|
|||
|
|
the argument to the <code class="docutils literal notranslate"><span class="pre">start_routine()</span></code> function. That is, when the thread is
|
|||
|
|
created, it begins executing as if the programmer had called <code class="docutils literal notranslate"><span class="pre">start_routine</span>
|
|||
|
|
<span class="pre">(arg);</span></code> in their code. The only thing that is different is that the function
|
|||
|
|
will begin executing in a different thread.</p>
|
|||
|
|
<p>The prototype for the <code class="docutils literal notranslate"><span class="pre">pthread_create()</span></code> function can be rather difficult to
|
|||
|
|
read if you are not completely comfortable with <a class="reference internal" href="Glossary.html#term-function-pointer"><span class="xref std std-term">function pointers</span></a>. Specifically, this prototype declares that the third
|
|||
|
|
parameter must be a pointer to a function that adheres to the following type
|
|||
|
|
declaration:</p>
|
|||
|
|
<div class="highlight-c border border-dark rounded-lg bg-light px-2 mb-3 notranslate"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="kt">void</span> <span class="o">*</span><span class="nf">start_routine</span> <span class="p">(</span><span class="kt">void</span> <span class="o">*</span><span class="n">args</span><span class="p">);</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</div>
|
|||
|
|
<p>This prototype requirement simplifies the interface specification for thread
|
|||
|
|
creation. The trade-off is that passing parameters becomes more complicated. The
|
|||
|
|
following program illustrates how to create a child thread and wait for it to finish.</p>
|
|||
|
|
<div class="highlight-c border border-dark rounded-lg bg-light px-0 mb-3 notranslate" id="cl6-2"><table class="highlighttable"><tr><td class="linenos px-0 mx-0"><div class="linenodiv"><pre class="mb-0"> 1
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9
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26
|
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27
|
|||
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28
|
|||
|
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29</pre></div></td><td class="code"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="cm">/* Code Listing 6.2:</span>
|
|||
|
|
<span class="cm"> A POSIX thread version of "Hello world"</span>
|
|||
|
|
<span class="cm"> */</span>
|
|||
|
|
|
|||
|
|
<span class="cp">#include</span> <span class="cpf"><stdio.h></span><span class="cp"></span>
|
|||
|
|
<span class="cp">#include</span> <span class="cpf"><pthread.h></span><span class="cp"></span>
|
|||
|
|
<span class="cp">#include</span> <span class="cpf"><assert.h></span><span class="cp"></span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Function that will run in the child thread */</span>
|
|||
|
|
<span class="kt">void</span> <span class="o">*</span>
|
|||
|
|
<span class="nf">start_thread</span> <span class="p">(</span><span class="kt">void</span> <span class="o">*</span><span class="n">args</span><span class="p">)</span>
|
|||
|
|
<span class="p">{</span>
|
|||
|
|
<span class="n">printf</span> <span class="p">(</span><span class="s">"Hello from child</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
|
|||
|
|
<span class="n">pthread_exit</span> <span class="p">(</span><span class="nb">NULL</span><span class="p">);</span>
|
|||
|
|
<span class="p">}</span>
|
|||
|
|
|
|||
|
|
<span class="kt">int</span>
|
|||
|
|
<span class="nf">main</span> <span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">**</span><span class="n">argv</span><span class="p">)</span>
|
|||
|
|
<span class="p">{</span>
|
|||
|
|
<span class="n">pthread_t</span> <span class="n">child_thread</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Create a child thread running start_thread() */</span>
|
|||
|
|
<span class="n">assert</span> <span class="p">(</span><span class="n">pthread_create</span> <span class="p">(</span><span class="o">&</span><span class="n">child_thread</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">,</span> <span class="n">start_thread</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">)</span>
|
|||
|
|
<span class="o">==</span> <span class="mi">0</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Wait for the child to finish, then exit */</span>
|
|||
|
|
<span class="n">pthread_join</span> <span class="p">(</span><span class="n">child_thread</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">);</span>
|
|||
|
|
<span class="n">pthread_exit</span> <span class="p">(</span><span class="nb">NULL</span><span class="p">);</span>
|
|||
|
|
<span class="p">}</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</td></tr></table></div>
|
|||
|
|
<p><a class="reference external" href="POSIXThreads.html#cl6-2">Code Listing 6.2</a> illustrates a subtle point that is not immediately
|
|||
|
|
obvious to novices: <strong>The ``main()`` function defines a thread.</strong> Specifically,
|
|||
|
|
every program begins with a single thread of execution with <code class="docutils literal notranslate"><span class="pre">main()</span></code> as its
|
|||
|
|
entry point. Once the pthread library creates an additional thread, it is
|
|||
|
|
important that <code class="docutils literal notranslate"><span class="pre">main()</span></code> should be treated as a thread. In the simplest terms,
|
|||
|
|
this perspective implies that <code class="docutils literal notranslate"><span class="pre">main()</span></code> needs to end with <code class="docutils literal notranslate"><span class="pre">pthread_exit()</span></code>
|
|||
|
|
instead of the standard return statement. Using <code class="docutils literal notranslate"><span class="pre">pthread_exit()</span></code> ensures that
|
|||
|
|
all threads are managed and run correctly.</p>
|
|||
|
|
<p>The main thread uses <code class="docutils literal notranslate"><span class="pre">pthread_join()</span></code> function to wait for the thread running
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">start_thread()</span></code> to call <code class="docutils literal notranslate"><span class="pre">pthread_exit()</span></code>. The standard thread terminology
|
|||
|
|
uses the term <a class="reference internal" href="Glossary.html#term-fork-join-pattern"><span class="xref std std-term">joining</span></a> to mean waiting on a thread to complete. When a
|
|||
|
|
thread exits, any resources associated with it (such as its run-time stack)
|
|||
|
|
remain allocated until the thread is joined. As such, the <code class="docutils literal notranslate"><span class="pre">pthread_join()</span></code> and
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">pthread_exit()</span></code> combination can be used to return a value from the exiting
|
|||
|
|
thread to its parent. In this example, nothing is returned, so both of these
|
|||
|
|
functions take NULL as an argument.</p>
|
|||
|
|
<p>Creating threads with <code class="docutils literal notranslate"><span class="pre">pthread_create()</span></code> happens <a class="reference internal" href="Glossary.html#term-asynchronous"><span class="xref std std-term">asynchronously</span></a>. That
|
|||
|
|
is, <code class="docutils literal notranslate"><span class="pre">pthread_create()</span></code> requests the allocation of resources for a new thread
|
|||
|
|
and returns 0 if the request is successful. <strong>The new thread may or may not
|
|||
|
|
begin running by the time ``pthread_create()`` returns.</strong> In fact, the new
|
|||
|
|
thread may have already run to completion by the time <code class="docutils literal notranslate"><span class="pre">pthread_create()</span></code>
|
|||
|
|
returns! The timing for running both the new and existing threads are determined
|
|||
|
|
by the system (the pthread library and the kernel). From the programmer’s
|
|||
|
|
perspective, this choice is nondeterministic.</p>
|
|||
|
|
<div class="topic border border-dark rounded-lg alert-danger px-2 mb-3">
|
|||
|
|
<div class="figure align-left">
|
|||
|
|
<a class="reference internal image-reference" href="_images/CSF-Images-BugWarning.png"><img alt="Decorative bug warning" src="_images/CSF-Images-BugWarning.png" style="width: 90%;" /></a>
|
|||
|
|
</div>
|
|||
|
|
<p class="topic-title first pt-2 mb-1">Bug Warning</p><hr class="mt-1" />
|
|||
|
|
<p>There are a couple of common mistakes with pthread function parameters. With
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">pthread_create()</span></code>, the first parameter must point to a <code class="docutils literal notranslate"><span class="pre">pthread_t</span></code>
|
|||
|
|
instance. It is common to (incorrectly) declare the variable as a
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">pthread_t*</span></code>. Instead, the variable should be a <code class="docutils literal notranslate"><span class="pre">pthread_t</span></code> and the address
|
|||
|
|
of it should be passed as shown above. Similarly, the first parameter for
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">pthread_join()</span></code> must be a <code class="docutils literal notranslate"><span class="pre">pthread_t</span></code> instance, not a pointer. Next, the
|
|||
|
|
third parameter to <code class="docutils literal notranslate"><span class="pre">pthread_create()</span></code> must be the name of the function and
|
|||
|
|
must not include (). The parentheses would indicate that the function should be
|
|||
|
|
called (before creating the thread) and the function’s return value specifies
|
|||
|
|
the address to start at in the new thread. The following code sample
|
|||
|
|
illustrates these bugs.</p>
|
|||
|
|
<div class="highlight-c border border-dark rounded-lg bg-light px-0 mb-3 notranslate"><table class="highlighttable"><tr><td class="linenos px-0 mx-0"><div class="linenodiv"><pre class="mb-0">1
|
|||
|
|
2
|
|||
|
|
3
|
|||
|
|
4
|
|||
|
|
5
|
|||
|
|
6</pre></div></td><td class="code"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="cm">/* WRONG! thread should not be a pointer in this example */</span>
|
|||
|
|
<span class="n">pthread_t</span> <span class="o">*</span><span class="kr">thread</span><span class="p">;</span>
|
|||
|
|
<span class="cm">/* WRONG! thread is invalid pointer and start should not have () */</span>
|
|||
|
|
<span class="n">pthread_create</span> <span class="p">(</span><span class="kr">thread</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">,</span> <span class="n">start</span> <span class="p">(),</span> <span class="nb">NULL</span><span class="p">);</span>
|
|||
|
|
<span class="cm">/* WRONG! pthread_join() doesn't take pointer as first parameter */</span>
|
|||
|
|
<span class="n">pthread_join</span> <span class="p">(</span><span class="kr">thread</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">);</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</td></tr></table></div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<div class="section" id="attached-and-detached-threads">
|
|||
|
|
<h2>6.4.2. Attached and Detached Threads<a class="headerlink" href="POSIXThreads.html#attached-and-detached-threads" title="Permalink to this headline">¶</a></h2>
|
|||
|
|
<p>In some scenarios, a thread will be created as <a class="reference internal" href="Glossary.html#term-detached-thread"><span class="xref std std-term">detached</span></a> instead of
|
|||
|
|
joinable. When a detached thread exits, its resources are immediately reclaimed
|
|||
|
|
for more efficient reuse. However, once a thread is detached, it can never be
|
|||
|
|
joined. This means that no other thread can wait on it to finish. According to
|
|||
|
|
the pthread specification, all threads are supposed to be created as joinable by
|
|||
|
|
default. The <code class="docutils literal notranslate"><span class="pre">pthread_attr_setdetachstate()</span></code> function can be used to change
|
|||
|
|
the <code class="docutils literal notranslate"><span class="pre">attr</span></code> argument to <code class="docutils literal notranslate"><span class="pre">PTHREAD_CREATE_DETACHED</span></code> to override the default.</p>
|
|||
|
|
<div class="highlight-c border border-dark rounded-lg bg-light px-0 mb-3 notranslate" id="cl6-3"><table class="highlighttable"><tr><td class="linenos px-0 mx-0"><div class="linenodiv"><pre class="mb-0">1
|
|||
|
|
2
|
|||
|
|
3
|
|||
|
|
4
|
|||
|
|
5
|
|||
|
|
6
|
|||
|
|
7</pre></div></td><td class="code"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="n">pthread_attr_t</span> <span class="n">attr</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Get the default set of attributes */</span>
|
|||
|
|
<span class="n">pthread_attr_init</span><span class="p">(</span><span class="o">&</span><span class="n">attr</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Mark the thread as detached */</span>
|
|||
|
|
<span class="n">pthread_attr_setdetachstate</span><span class="p">(</span><span class="o">&</span><span class="n">attr</span><span class="p">,</span> <span class="n">PTHREAD_CREATE_DETACHED</span><span class="p">);</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</td></tr></table></div>
|
|||
|
|
<p>Alternatively, the <code class="docutils literal notranslate"><span class="pre">pthread_detach()</span></code> function can be used to detach a running thread instance. A thread can detach itself by using its own <code class="docutils literal notranslate"><span class="pre">pthread_t</span></code> identifier as an argument to <code class="docutils literal notranslate"><span class="pre">pthread_detach()</span></code>.</p>
|
|||
|
|
<div class="highlight-c border border-dark rounded-lg bg-light px-2 mb-3 notranslate" id="cl6-4"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="n">pthread_detach</span> <span class="p">(</span><span class="n">pthread_self</span> <span class="p">());</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</div>
|
|||
|
|
<div class="topic border border-dark rounded-lg bg-light px-2 mb-3">
|
|||
|
|
<div class="figure align-left">
|
|||
|
|
<a class="reference internal image-reference" href="_images/CSF-Images-Library.png"><img alt="Decorative C library image" src="_images/CSF-Images-Library.png" style="width: 100%;" /></a>
|
|||
|
|
</div>
|
|||
|
|
<p class="topic-title first pt-2 mb-1">C library functions – <pthread.h></p><hr class="mt-1" />
|
|||
|
|
<dl class="docutils">
|
|||
|
|
<dt><code class="docutils literal notranslate"><span class="pre">int</span> <span class="pre">pthread_detach</span> <span class="pre">(pthread_t</span> <span class="pre">thread);</span></code></dt>
|
|||
|
|
<dd>Detaches the specified thread.</dd>
|
|||
|
|
<dt><code class="docutils literal notranslate"><span class="pre">pthread_t</span> <span class="pre">pthread_self</span> <span class="pre">(void);</span></code></dt>
|
|||
|
|
<dd>Returns the current thread’s pthread_t identifier.</dd>
|
|||
|
|
</dl>
|
|||
|
|
</div>
|
|||
|
|
<div
|
|||
|
|
id="POSIXThreadSumm"
|
|||
|
|
class="embedContainer"
|
|||
|
|
data-exer-name="POSIXThreadSumm"
|
|||
|
|
data-long-name="POSIX thread questions"
|
|||
|
|
data-short-name="POSIXThreadSumm"
|
|||
|
|
data-frame-src="../../../Exercises/Threads/POSIXThreadSumm.html?selfLoggingEnabled=false&localMode=true&module=POSIXThreads&JXOP-debug=true&JOP-lang=en&JXOP-code=java"
|
|||
|
|
data-frame-width="950"
|
|||
|
|
data-frame-height="550"
|
|||
|
|
data-external="false"
|
|||
|
|
data-points="1.0"
|
|||
|
|
data-required="True"
|
|||
|
|
data-showhide="show"
|
|||
|
|
data-threshold="3"
|
|||
|
|
data-type="ka"
|
|||
|
|
data-exer-id="">
|
|||
|
|
|
|||
|
|
<div class="center">
|
|||
|
|
<div id="POSIXThreadSumm_iframe"></div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
|
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
|
|
|||
|
|
|
|||
|
|
<div class="container">
|
|||
|
|
|
|||
|
|
<div class="mt-4 container center">
|
|||
|
|
«  <a id="prevmod1" href="RaceConditions.html">6.3. Race Conditions and Critical Sections</a>
|
|||
|
|
  ::  
|
|||
|
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<a class="uplink" href="index.html">Contents</a>
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  ::  
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<a id="nextmod1" href="ThreadArgs.html">6.5. Thread Arguments and Return Values</a>  »
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